The present invention generally relates to photolithography, and more specifically relates to methods and apparatuses for increasing resolution limits in photolithography.
Microlithography is used to manufacture integrated circuits, magnetic devices, and other microdevices. In microlithography, a final product is manufactured in a multiple step process, where initially a “resist” material is produced with each pattern subsequently defining a product attribute. “Resists” are generally formed of polymer compositions, and are sensitive to light or other forms of radiation. The patterns are formed in the resist by exposing different regions of the resist material to different radiation doses. In bright regions, chemical changes occur in the resist that cause it to dissolve more easily (for positive resist) or less easily (for negative resists) than in dim regions. The bright and dim regions are exposed using an exposure tool which generally transfers corresponding features from a mask or reticle. The masks or reticles are generally plates of quartz coated with an opaque material such as chrome. The chrome is etched away to form the mask. The radiation used may be, for example, ultraviolet light and x-rays, and the regions of the mask that are opaque and transparent form a pattern of bright and dark when illuminated uniformly.
Typically, a projection lens is used to form an image of the mask pattern on the resist film. The patterns formed in the resist are not identical to those on the mask, and the methods of obtaining the pattern desired for the ultimate manufactured device in spite of deficiencies in the process is called “wavefront engineering.” This includes Optical and Process Correction or Optical Proximity Correction (OPC), wherein edge placements are manipulated, and off-axis illuminations. Among the various devices used are phase shift masks (PSMs), which create desired dark regions through interference. Presently, two types of phase shift masks are in use: weak-PSMs and strong-PSMs, such as Alternating-Aperture-PSMs. These two differ in that the weak-PSMs have only one type of bright feature, while the strong-PSMs contain two types of bright features identical except for the optical phase, which differs by 180 degrees. Phase shift masks and their use in photolithography are described in detail in several existing documents, including U.S. Pat. Nos. 5,620,816; 5,807,649; 6,251,549; 6,287,732 and 6,479,196, all of which are incorporated herein by reference in their entirety.
Although phase shift masks and their use in photolithography provide distinct advantages, improvements can be made with regard to resolution, and the present invention is directed at improving resolution limits when using masks in a photolithography process, such as when using, for example, Alternating-Aperture-PSMs or Alternating-PSMs.
In the use of Alternating-PSMs, to produce a target layout 10 as illustrated in FIG. 1, a phase mask 12 as illustrated in FIG. 2 is formed where the phase mask includes Phase1 14 and Phase2 16, and a binary trim/field mask 18 as illustrated in FIG. 3 is formed, where the binary trim/field mask 18 includes trim patterns 20. For double exposure Alternating-PSM technology, full resolution enhancement potential of the second mask, i.e., the binary trim/field mask (see FIG. 3) is not generally possible. Different exposure conditions for the two masks (i.e., the phase mask 12 as shown in FIG. 2 and the binary trim/field mask 18 as shown in FIG. 3) reduce throughput, and the binary trim/field mask 18 is difficult to image due to near-resolution-limit binary mask exposure.